The present invention relates generally to monolithic microwave integrated circuits, and more particularly, to a monolithic microwave integrated circuit module fabricated using tin based (Pb/Sn) solder technology and a method of fabricating same.
Flip chip monolithic microwave integrated circuit with plated silver thermal bumps are attached to microwave module substrates with gold based metallization using solder reflow techniques. Dielectric solder dams are provided on the metal patterns of the substrate to confine the spread of solder during the reflow process. Lead/indium solder is used to avoid the potential reliability hazard associated with gold/tin intermetallics.
The prior art is incompatible with tin based (Pb/Sn) solder techniques, and thus indium solder technology has been used for MMIC flip chip attachment. There is no commercial technique to plate indium solder, a process critical to millimeter-wave flip chip assemblies. Also, the use of gold based circuitry on an insulating substrate is more costly than the use of copper based circuitry. Pattern definition of dielectric (glass) solder dams also poses a lower limit on usable bump dimensions. Bump size reduction is critical to flip chip MMIC die size and module cost reduction.
Gold based circuitry is commonly used in both monolithic microwave integrated circuits (MMICs) and hybrid microwave assemblies because of the corrosion resistance property of gold. The same metal scheme is frequently used in traditional flip chip MMICs and flip chip microwave assemblies. Gold is also compatible with gold bond wires for off-chip electrical interconnections.
Indium based solder (i.e. 50/50 Pb/In) is used in place of more common tin based solder for these microwave assemblies because of reliability concerns of gold-tin compounds with high gold content. In particular, the gold-tin compound can become brittle, which is a potential cause of solder joint failure. In addition, dielectric (i.e. glass, polyimide, etc.) solder dams are needed to prevent solder from spreading on the surface of the microwave integrated circuit substrate during solder reflow. Therefore, lead indium (Pb/In) solder, which is more compatible with gold based metallization, is generally used for flip chip MMIC attachment.
Development of lower cost, higher frequency (millimeter wave), high power MMIC flip chip attachment techniques requires the use of chips with plated solder caps. Lack of Pb/In solder plating technology hinders the advancement of flip chip monolithic microwave integrated circuits, the most promising microwave module manufacturing technology of the future.
Accordingly, it is an objective of the present invention to provide for an improved monolithic microwave integrated circuit module fabricated using tin based (Pb/Sn) solder technology and a method of fabricating same.